Method of manufacturing formed objects from a chromium-carbon-iron alloy

ABSTRACT

Formed objects of high strength even at higher temperatures are obtained by unidirectional solidification of an iron-chromiumcarbon alloy in a given range of compositions around a straight line in the triangular phase diagram connecting points representing the compositions chromium 15 percent by weight, carbon 4 percent by weight, remainder iron and chromium 30.5 percent by weight, carbon 2.96 percent by weight, remainder iron.

Van Den Boom gaard et al.

[4 1 Jan. 15, 1974 METHOD OF MANUFACTURING FORMED OBJECTS FROM A CHROMIUM-CARBON-IRON ALLOY Inventors: Jan Van Den Boonigaard; Lodewijk Reinaerd Wolff, both of Emmasingel, Eindhoven, Netherlands U.S. Philips Corporation, New York, N.Y.

Filed: Apr. 5, 1971 Appl. No.: 131,477

Assignee:

Foreign Application Priority Data Apr. 3, 1970 Netherlands 7004761 U.S. Cl 75/130.5, 75/126 A, 75/135 Int. Cl. C22c 39/14 Field of Search 75/130.5, 135, 126 A References Cited UNITED STATES PATENTS I 11/1917 Becket 75/126 A 3,671,223 6/1972 Thompson et al. 75/122 2,276,287 3/1942 Chandler 75/130.5 X 2,364,257 12/1944 Vetter 75/130.5 3,180,727 4/1965 Alexander et al. 75/135 X 3,564,940 2/1971 Thompson et a1. 75/134 Primary ExaminerL. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Att0rneyFrank R. Trifari [57] ABSTRACT Formed objects of high strength even at higher temperatures are obtained by unidirectional solidification of an iron-chromium-carbon alloy in a given range of compositions around a straight line in the triangular phase diagram connecting points representing the compositions chromium 15 percent by weight, carbon 4 percent by weight, remainder iron and chromium 30.5 percent by weight, carbon 2.96 percent by weight, remainder iron.

2 Claims, No Drawings METHOD OF MANUFACTURING FORMED OBJECTS FROM A CHROMIUM-CARBON-IRON ALLOY The invention relates to a method of manufacturing formed objects of an alloy reinforced by faceted, needle-shaped crystals by unidirectional solidification of the melt of an alloy.

Methods of this kind are known from literature.

Unidirectional solidification may increase the strength of a material, which may be advantageous.

However, actual reinforcement depends upon a number of factors:

a. The volume fraction of the needle-shaped crystals in the matrix has to be large.

b. The needle-shaped crystals should have a comparatively high length-diameter ratio with a small diameter. Y

c. The needle-shaped crystals possibly have to be single crystals.

d. The needle-shaped crystals have to be of a material of high strength.

From a phase diagram it cannot be deduced that in using an unidirectional solidification process needleshaped crystals will be formed nor that needle-shaped crystals, if formed, will influence the mechanical properties of the material to an extent such that usable material is obtained.

The invention has for its object to provide a method of manufacturing formed objects consisting of an, iron alloy reinforced by faceted, needle-shaped crystals.

It has been found that formed objects, possessing great strength even at temperatures of 600 C and more, can be obtained if an alloy comprising iron, chromium and carbon is unidirectionally solidified if the composition of the alloy is given in the triangular phase diagram by a point inside a rectangle having as angular points the following compositions: chromium 13.5 percent by weight, carbon 3.4 percent by weight, remainder iron; chromium 30.5 percent by weight, carbon 2.4 percent by weight, remainder iron; chromium 33 percent by weight, carbon 3.4 percent by weight, remainder iron and chromium 16 percent by weight, carbon 4.4 percent by weight, remainder iron.-

Apart from chromium and carbon in quantities as defined by the rectangle the alloy contains solely iron with the usual impurities such as cobalt, copper, molyb denum, aluminium, nickel, magnesium, silicon, manganese, sulphur, phosphorus. The unidirectionally solidified material is found to consist of a matrix of 01- and/or 'y-iron traversed by parallel, needle-shaped, hexagonal crystals of iron-containing chromium carbide: (Cr,Fe)-,C;,. Apart from small quantities of carbon the matrix may comprise up to percent by weight of chromium dissolved in it. The chromium carbide phase may comprise up to about 30 percent by weight of iron.

Inside the rectangle as defined above a line connects the points defining the compositions: chromium percent by weight, carbon 4 percent by weight, remainder iron and chromium 30.5 percent by weight, carbon 2.96 percent by weight, remainder iron.

. When the composition of the alloy is chosen so that it is given by a point on said line, the unidirectional solidification is found to provide a product of regular structure containing substantially long, needle-shaped chromium carbide crystals having diameters up to p" lf starting with a composition given by a point on said line the carbon content is reduced, whereas the chromium content remains the same, compositions are obtained in which iron dendrites are usually formed if such compositions are unidirectionally solidified. The formation of iron dendrites has to be avoided as far as possible. Bending and stretching tests have shown that such dendrites may constitute weak places in the objects.

lf starting with a composition given by a point on the line the carbon content is raised with the chromium content remaining constant compositions are obtained in which needle-shaped chromium carbide crystals with larger diameter are formed during unidirectional solidification the orientation of which usually differs from that of the fine needle-shaped crystals. These thicker needle-shaped crystals may diminish the strength of the formed objects by their differing orientation. Within the limits as defined by the rectangle for the chromiumcarbon content it is found to be possible, however, by an appropriate choice of the temperature gradient at right angles to the solidfication front and of the rate at which the solidification front is moved across the formed object, to avoid the formation of iron dendrites and chromium carbide crystals of undesirable diameter. Outside said region in the triangular phase diagram the rate of solidification becomes too low and the temperature gradient becomes too high to obtain this result. If the composition of the alloy is chosen within a narrow region around the line connecting the points defining the compositions chromium 15 percent by weight, carbon 4 percent by weight, remainder iron, and chromium 30.5 percent by weight, carbon 2.96 percent by weight, remainder iron, the regularity of the structure of the product of the unidirectional solidification is independent within comparatively wide limits of the rate of solidification and the temperature gradient used. This means that with compositions defined by points lying on said line the solid phases: solid y-Fe and solid chromium carbide Cr,Fe C are formed without a variation of the temperature and the average composition of the melt in this process.

The preferred range of composition was determined by examining rapidly solidified drops.

The last-mentioned preferred range of compositions is indicated by a rectangle in the phase triangle having as angular points the compositions: chromium 15.5 percent by weight, carbon 4.2 percent by weight, remainder iron, chromium 31 percent by weight, carbon 3.2 percent by weight, remainder iron; chromium 30 percent by weight, carbon 2.7 percent by weight, remainder iron and chromium 14.5 percent by weight, carbon 3.8 percent by weight, remainder iron.

The method according to the invention permits using the conventional techniques for orientated solidification, for example, the Bridgman or Czochralski method, the zone-melting technique or the floatingzone technique.

Formed objects of high mechanical strength are also obtained by unidirectional solidifying the alloy in a radial direction.

In the following examples iron is used, which is obtained by heating sintered pellets of carbonyl iron in a hydrogen atmosphere (76 cms l-lg) at 900 C for 45 minutes and by pulverizing the material subsequent to cooling. It comprises as impurities usually 0.04 percent by weight of Ni, 0.002 percent by weight Mn, 0.0004 percent by weight Al, 0.0005 percent by weight C and less than 0.00001 percent by weight Si. The carbon used is a so-called spectral quality of high purity. The chromium employed contains 0.5 percent by weight Si, 0.01 percent by weight Al, 0.001 percent by weight Ca,

The molten zone is subsequently displaced downwardly through the bar at a rate of 1 mm a minute. The bar portion located above the molten zone was caused to rotate at a speed of 90 revolutions a minute. After 0.06 percent by weight Mn, 0.007 percent by weight 5 the melting zone has passed through the bar, the latter Mg, 0.01 percent by weight Pb and 0.02 percent by is annealed at a temperature of 900 C for 2 hours and weight Ni. then cooled in eight hours to about 20 C.

After the unidirectional solidification the resulting Examination showed that bars obtained as described objects may be subjected to a thermal treatment for in this Example consisted of a matrix of aand 'y-iron raising the hardness. The piece is heated at a temperaand fine chromium carbide needles embeddedin the ture (for example, 950 C), at which the available bar in the longitudinal direction. a-iron is converted into -ii-on. Subs uentl the The tensile strength of the material up to rupture was formed piece is quenched, for example, in a w terdetermined on the bars (diameter 3 mms) obtained in cool d oil b th, accordance with this Example at different tempera- The invention will be d rib d mo f ll ith f tures: see the following Table 2. The measurements ence to the following Examples. were carried out in air:

Example I TABLE II The preferred region of composition was determined as follows: In an argon atmosphere of a pressure of Tensile strength in (g/mm, about 25 cms a mixture of iron and carbon is melted on T in C a, M a copper plate constantly cooled with water by striking 20 93 i 3 an electric are between a tungsten electrode and the 320 8813 mixture. Then the electric arc is switched off and the 640 8:3 calculated quantity of chromium is added. An arc is 925 30:4 again struck until the mixture has melted. The are is switched off. After the melt has solidified. the pellet ob- E l [I] tained in this way is turned over and again melted in the A d ib d i E l 11 b f a di f 3 manner described. This process is repeated a few times mms are made from an alloy consisting of: until a homogeneous melt is obtained (usually three 73.7 percent by weight of iron times is found to be sufficient). 3.5 percent by weight of carbon and From the body thus obtained a microsection is made 22.8 percent by weight of chromium. and examined under a microscope; in addition, the The molten zone is passed at a rate of 2 mms a minhardness was measured and an X-ray analysis was carute across the bar. ried out. The results mentioned in the following Table The bars thus manufactured consist of aand 'y-iron were obtained: in which fine chromium carbide needles are embedded V" My 7 WW WM TABLE I if '7 i 7 Comp. in by w. Presence of: Structural details Hardness in Vickers C Fe needles (CT, b C3 Cl'ziC F63 C 15 4 remainder fine 800-900 16 4 remainder less fine 800-900 15 3.5 remainder Fe-dendrites 700-800 21.5 3.5 remainder fine 800-900 22 4 remainder coarse carbide needles 600-700 27.75 3.25 remainder fine 800-900 30 3 remainder fine 800-900 30 2.5 remainder Fe-dendrites 600-700 30 3.5 remainder coarse carbide needles 700-800 In this Table and the Examples fine means that the carbide needles have a thickness of less than 20 pm: coarse means that the thickness is more than 20pm up to 200 pm, absent present.

Example I] 74 gs of iron are melted in an alumina crucible in an argon atmosphere (pressure: 76 cms l-lg.) To the melt are added 3 gs of carbon. After about 5 minutes the whole quantity of carbon is dissolved in the iron. Then 23 gs of chromium are added.

After about 10 minutes it is found that a homogeneous melt is obtained. Then bars are cast which have a diameter of 3 mms (mould of quartz).

A bar thus obtained is arranged in vertical position in a floating-zone apparatus, which is then filled with hydrogen at a pressure of 76 cms Hg. ln the upper portion of the bar a narrow zone is heated at a temperature exceeding the melting point of the alloy.

in the direction of length of the bar.

The tensile strength measured in air on the bar upto rupture amounts to:

As described in Examples 11 bars of a diameter of 3 mms are made from an alloy consisting of 73.8 percent by weight of iron, 3.25 percent by weight of carbon and 22.95 percent by weight of chromium.

The molten zone is passed in this case at a rate of 0.7 mm a minute across the bar.

The bars thus manufactured consist of aand y-iron in which tine chromium-carbide needles are embedded in the direction of length of the bar.

The tensile strength measured on this bar in air up to rupture amounts to (Table IV).

TABLE IV Tensile strength in kg/mm T in C Example V TABLE V ELmodule in lag/mm T in "C The bars made in accordance with this Example consist of aand 'y-iron in which fine chromium-carbide needles are embedded and oriented in the longitudinal direction of the bar.

Example V1 As described in Example III bars of a diameter of 5 mms are made from an alloy consisting of 67 percent by weight of iron, 3 percent by weight of carbon and 30 percent by weight of chromium.

The molten zone is in this case passed at a rate of 5 mms a minute through the bar.

The elasticity module of the alloy in the direction of length of the bar is dynamically determined at different temperatures in an argon atmosphere (76 cms Hg), see

The bars made in accordance with this Example consist of aand 'y-iron in which fine chromium-carbide needles are embedded and oriented in the longitudinal direction of the bar. Example VII As described in Example 1 bars are made with a diameter of 3 mms from an alloy consisting of:

74.3 percent by weight of iron,

3.6 percent by weight of carbon and 22.1 percent by weight of chromium.

The molten zone is passed in this case at a rate of 1.5 mms a minute through the bar. A structure of needleshaped crystals of chromium-carbide orientated in the longitudinal direction of the bar in a matrix of aand 'y-iron is obtained.

The tensile strength up to rupture of the bars was at about 20 C: 74 kgs/mm Example VIII Bending tests (three-point bending test) on bars ground off center-less to a diameter of 1.95 mms of alloys as indicated in Examples II to VII shown that the unidirectionally solidified material has a strength at least twice that of a non-unidirectionally solidifed material. In the latter case the resistance to rupture is about 15 kgs at about 20 C.

In the unidirectionally solidified material the fracture extends partly in the longitudinal direction of the bar and approximately at right angles to the direction of the load. With non-unidirectionally solidified material the plane of fracture is substantially at right angles to the direction of length of the bar.

The method according to the invention has the particular advantage that alloys of the comparatively cheap iron can provide formed objects of high strength. Even at higher temperatures this strength is maintained for the major part. By the method according to the invention, chisels, drills, cutters and other tools of high strength can be made, which remain usable up to comparatively high temperatures.

What is claimed is:

l. A method of manufacturing formed objects from an iron alloy free of dendrites and reinforced by faceted, needle-shaped crystals by unidirectional solidification of a melt of an alloy comprising the steps of forming an alloy of iron, chromium and carbon the composition of which is given by a point in the triangular phase diagram inside a rectangle having as angular points the compositions: 13.5 percent by weight of chromium, 3.4 percent by weight of carbon, remainder iron; chromium 30.5 percent by weight, carbon 2.4 percent by weight, remainder iron; chromium 33 percent by weight, carbon 3.4 percent by weight and remainder iron and chromium 16 percent by weight, carbon 4.4 percent by weight and remainder iron, and unidirectionally solidifying said alloy to form therein substantially long, needle-shaped chromium carbide crystals having diameters up to 20p" 2. A method as claimed in claim 1 characterized in that the alloy comprising iron, chromium and carbon which is unidirectionally solidified has a composition which is given by a point of the triangular phase diagram inside a rectangle having as angular points the compositions: chromium 15.5 percent by weight, carbon 4.2 percent by weight, remainder iron; chromium 31 percent by weight, carbon 3.2 percent by weight, remainder iron; chromium 30 percent by weight, carbon 2.7 percent by weight, remainder iron and chromium 14.5 percent by weight, carbon 3.8 percent by weight,

remainder iron.

7%? UNITED STATES PATENT m m CERTIFICATE 0 cnmm Patent No. ,78 ,805 Dated January 15 1 974 Inventor(s) JAN VAN DEN BOOMGAARD ET AL It is certified that error appears in the above-identified patent and that said Letters Patent: are hereby corrected as shown below:

In TABLE I, change the sub-headings from needles (Cr, Fe) C (Ii-" C Fe (3" to read -needles (Cr, Fe)7C- Cr C Fe3C mg Fe d -Fe-m Signed and sealed this 28th day ef January 1975.

(SEAL) Attest: I

mm: M; GIBSON JR; c. MARHA L mm Attesting Officer Coimniseiener of Patents mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORECTION Patent NO. 1'7 51 I fl JAN VAN DEN :u: i

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the title page section [30] change the priority application number to read -7004767--.

Signed and sealed this 22nd day of April 1975.

(SEAL) Attest C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks 

2. A method as claimed in claim 1 characterized in that the alloy comprising iron, chromium and carbon which is unidirectionally solidified has a composition which is given by a point of the triangular phase diagram inside a rectangle having as angular points the compositions: chromium 15.5 percent by weight, carbon 4.2 percent by weight, remainder iron; chromium 31 percent by weight, carbon 3.2 percent by weight, remainder iron; chromium 30 percent by weight, carbon 2.7 percent by weight, remainder iron and chromium 14.5 percent by weight, carbon 3.8 percent by weight, remainder iron. 